The invention relates to energy measurement for a lighting system, for example a lighting system used in an office building. European patent application EP2838322 discloses a method and apparatus for energy monitoring in lighting systems. This known method requires a power calibration step in which each of the lighting devices in an area is activated sequentially. This makes it easy to find out the power use associated with a particular lighting device but has the disadvantage that an artificial power calibration phase is needed to obtain the energy use of the lights. The known method advises to perform the power calibration only occasionally and preferably at night.
A deterioration of the lighting system or a sudden condition will not be detected until the next power calibration step is carried out. In addition, this known method and apparatus have the disadvantage that energy is wasted by activating lights which are otherwise not needed, in particular if the power calibration step is carried out during the night or if the power calibration is done frequently. Furthermore, activating various lights in a building during the night may not be possible due to night uses of the building, and moreover may attract the attention of security personnel, thus causing false security alarms.